Multi-module Maven project support for SAST

Problem to solve

Multi-module projects are not yet supported for SAST. Scanning a project like WebGoat will return an error.

Further details

Multi-module projects are complex but more frequent than the simpler single module ones. This issue is about improving our Java support, and cover a lot more projects than what we have today.
Note that multi-module projects are already supported by Gemnasium (with possible edge-cases not yet discovered).

Proposal

Following discussions on #6733 (comment 84586601), it seems possible to leverage the FSB CLI to handle this kind of projects.